With development of economy and society, an energy crisis gradually emerges, and a global environment gradually deteriorates. Developing and using clean alternative energy has become an important objective in the energy industry. With continuous development of new energy power generation, energy storage, and new energy vehicle industry, a converter that is used as a core energy control apparatus becomes one of key factors in clean energy application. Among a variety of converters, a three-phase converter is one of the converters that are applied most widely, and is used to connect a three-phase alternating-current power system and a direct-current power system and transfer energy between the two systems. Two working states rectification and inversion are included based on different flowing directions of energy. In inversion, energy is transferred from the direct-current system to the alternating-current system. In rectification, energy is transferred from the alternating-current system to the direct-current system. Conversion efficiency and power quality are two key technical indicators of the three-phase converter. A modulation scheme directly affects an on/off state of a switch device, and therefore is one of the key factors that affect the conversion efficiency and the power quality of the three-phase converter.
A common pulse width modulation approach in a three-phase converter is Pulse Width Modulation (PWM). To be specific, widths of driving pulses of devices in a switch network are controlled. In a most direct implementation form, a carrier is compared with a modulation wave, and a comparison result is used to control an on/off state of a switch device. PWM may further include continuous pulse width modulation (Continuous Pulse Width Modulation, CPWM) and discontinuous pulse width modulation (Discontinuous Pulse Width Modulation, DPWM). DPWM has fewer switching times than CPWM, and therefore has a lower switching loss and can improve energy conversion efficiency of the converter. However, when the modulation scheme DPWM is used, a harmonic distortion factor is generally higher than that in the scheme CPWM, and content of injected harmonics is higher, causing system resonance more easily. In addition, when the three-phase converter has a lower modulation degree, content of injected harmonics in DPWM is higher. When the three-phase converter has a relatively high modulation degree, DPWM and CPWM have close injection levels. Therefore, DPWM needs to be used to ensure energy conversion efficiency when the three-phase converter has a relatively low modulation degree. In this case, it becomes a focus of research to improve a common-mode injection method of DPWM to reduce common-mode content injected in DPWM and reduce content of harmonics generated in common-mode injection.
Two existing technologies used to reduce content of harmonics generated in common-mode injection when a three-phase converter uses DPWM are as follows: 1. A rise rate of a common-mode voltage is limited to reduce harmonics generated due to rapid change of the common-mode voltage, to equivalently extend a change time of the common-mode voltage and shorten duration of a switching state of the three-phase converter. 2. A direct current bus voltage is used as a reference object, the direct current bus voltage is measured to calculate a clamping interval of a switching transistor and a corresponding turn-on angle, and an amplitude limiting stage is added. A common-mode modulation voltage corresponding to three phases is calculated based on the direct current bus voltage, the clamping interval of the switching transistor, and phase information. A corresponding final modulation voltage is calculated based on a fundamental-frequency sinusoidal modulation voltage and the common-mode modulation voltage of the three phases.
However, in Method 1, if an amplitude-limiting value is excessively small, duration of the switching state of the three-phase converter in DPWM cannot be significantly reduced, and the advantage of efficiency improvement is reduced. If an amplitude-limiting value is excessively large, it is difficult to suppress high-frequency harmonics, and implementation is insufficiently flexible. According to actual tests, in a three-phase converter, a magnitude of a common-mode modulation voltage is not determined by a direct current bus voltage. Therefore, Method 2 is not applicable to a scenario in which a three-phase converter uses DPWM. In addition, the calculation process is relatively complex, and the calculation consumes an excessively long time. Therefore, Method 2 is not applicable to a scenario of a three-phase converter that has relatively high switching frequency.